I'd get a card with a minimum write speed of 20mb/sec.WATCH OUT. Read the specs. Write speeds are 1/3 and less of the card speed, which uses read speeds.EXAMPLE: $70 SANDISK 45 mb/sec card has a 10mb/sec minimum read speed!The 64GB SDXC Memory Card Extreme Class 10 from SanDisk is a high-performance memory card. It improves upon the previous version's 30 MB/s max read / write speed by offering faster 45 MB/s performance. Minimum write speed is still 10 MB/s. These data rates get you ready for the next shot faster, and cut down on wait time when you're transferring your work from card to PC.

The maximum write speed is based on a totally erased (new) card. Formatting a card does not erase it unless its a low level format. Then the card has to first erase a block of data before it can write to it, and for a SD card, its SLOW.The 6D, like the 5D MK III will probably have a low level format function for SD cards which will totally erase a card, but it will take a long time, maybe 20-30 minutes depending on card size, so its best done in a computer where you don't run your battery down.